In order that the combustion, during the descent towards the ground of the illuminating charge, shall take place only at the end facing the ground, the other sides of the illuminating charge are usually enclosed by an insulating layer, which prevents ignition of these parts of the illuminating charge.
It is previously known to use for example epoxy plastic mixed with a filling agent (e.g. asbestos) as an insulating material. The insulation is then cast around the flare body, which requires comparatively much work and, accordingly, involves high production costs. Further, the components comprised in epoxy plastic are liable to cause allergy, with symptoms such as itching, swelling and headaches for the personnel.
Owing to, inter alia, said disadvantages, attempts have been made to use pasteboard cases for insulation of the illuminating charges. For reasons of strength, it has not been possible to press the illuminating charges direct into thin-walled pasteboard cases with high compacting pressures. This is due to the high friction forces which arise between the illuminating charge and the case when the illuminating charge is compacted, as the illuminating charge, which before the compacting consists of a powdered or granulated material, will be compacted at the high pressures to about one-half of its original volume, in order to give a low porosity and, accordingly, a high strength.
In order to solve this problem, the pasteboard cases have either been provided with excessively thick and hard walls which, however, have had a detrimental influence on the resulting quality of the cases and the burning properties, or else methods other than pressing have been resorted to for compacting of the pyrotechnical charge. Thus, according to the Swedish Patent Specification No. 6123/71, a method is known according to which the case is filled through stamping, rather than pressing. For stamping, however, more bonding agent is required, which is to the detriment of the performances of the illuminating charge. On the other hand, using less bonding agent will impair the strength.
Further, in the German published application No. 1 261 791, a method has been proposed of pressing a pyrotechnical charge into a pasteboard case without the case's being creased, by coating the inside of the case with a thin film of paper or plastic. This film creases during the pressing, and gives a low friction against the walls of the case, and therefore a pressed body with a homogeneous density can be obtained. However, the creases which become included have a detrimental influence on the strength, and cause a less uniform burning velocity.